In order to control the speed on the generating torque of a brushless DC motor (direct current motor), currents for a plurality of the stator coils can be switched and controlled electronically, usually by transistors. A brushless DC motor, which has neither mechanical brushes nor commutators, but has contactless electronic commutators such as transistors, has been designed and manufactured as a highly reliable motor. In conventional brushless DC motor, the total armature current is usually controlled by a transistor connected in series with the armature, and the current path to a plurality of stator coils is switched by commutating transistors operated in ON and OFF modes.
The commutating transistors can be used to control the magnitude of current in stator coils in the non-saturating mode. The armature current is controlled indirectly by controlling the base currents of the commutating transistors. The current flowing through each stator coil varies depending upon h.sub.FE (forward current transfer ratio) of each commutating transistor. Therefore it is inevitable that the total current flowing through the stator coils has ripples in the waveforms thereof.
An externally excited DC motor with a permanent magnet generates a torque essentially propotional to the armature current. Therefore, an unbalance or a difference in h.sub.FE of the commutating transistors causes a ripple in the generated torque.
U.S. Patent Specification No. 3,751,676 discloses an electronic control circuit which overcomes the abovesaid defects of the conventional circuit. The electronic control circuit in U.S. Patent Specification No. 3,751,676 forms a feedback loop wherein a current detector detects the total current flowing through the stator coils and the output of the current detector is used as a feedback signal so that the current for the stator coils is propotional to a reference signal inespective of an unbalance and a variation of h.sub.FE values of the commutating transistors, thus the torque ripple of the brushless DC motor due to the h.sub.FE unbalance has been remarkably reduced.
Recently, the electronic control circuit is easily constructed by a monolithic integrated circuit IC, and the number of circuit components has been remarkably reduced because an IC is just one electronic component even if it has a large number of transistors, diodes and resistors. h.sub.FE values of transistors in an IC sample are well matched. However, h.sub.FE values among IC samples in mass production vary so largely by a half or double from the nominal value. According to the h.sub.FE variation, the loop gain of the feedback loop varies greatly, and an oscillation sometimes occurs in a high h.sub.FE sample. Further, h.sub.FE of a transistor increases with the temperature increase.
Therefore, it is an essential problem how to compensate the feedback loop so as to avoid any oscillation due to the h.sub.FE variation. A good compensating method of the feedback loop is desired so that the 0 dB frequency, at which the absolute value of the open loop gain becomes 1 (0 dB), is constant irrespective of an unbalance and a variation of h.sub.FE values of transistors in mass production.